Laminate composite materials are becoming increasingly common in the constructions of large aircraft. Typical laminate composite materials are composed of layered resin bonded graphite textiles. Like any material disposed along the exterior of an aircraft, laminate composite materials are subject to damages during the service life of an aircraft. In-flight collisions with birds and air-borne debris, and ground collisions involving loading and maintenance vehicles and equipment cause visible impact sites along the exterior of an aircraft. Assessments and repairs of laminate composite materials represent significant challenges with regard to efficiencies in time, cost, and training. Ground maintenance crews may be able to recognize impact sites along aircraft exteriors, but are typically not sufficiently trained or equipped to assess damages that may be associated with impact sites.
Damages in laminate composite materials tend toward morphologies with two important characteristics. In one characteristic, damages tend to initiate at impact sites and propagate into structures in expanding cone patterns. In another characteristic, deeper delaminations are rarely present without corresponding shallow delaminations. Therefore, at any impact site with delamination damage, shallow damage is likely to be present and can be construed as an indication that deeper damage is likely to exist. Unfortunately, visible impact sites that represent mere superficial markings are not easily distinguished by visual inspection from those overlying significant internal damages. Non-destructive inspection (NDI) devices are available so that inspections can reveal hidden sub-surface damages. For example, graphical representations such as ultrasonic C-scan images and infrared thermographic images can reveal sub-surface problems. However, typical available technologies require considerable training and experience. Thus, typical available NDI approaches are in the practice domains of highly trained specialists.
However, a typical scenario faced by commercial airlines occurs when a ground maintenance crew member spots an impact site along the exterior of an aircraft as the craft is serviced between flights. A decision must be made as to whether the aircraft should be permitted to fly or should be grounded for thorough inspections, damage assessments, and repairs if necessary. Consequences can be severe when such a decision is poorly made. Both safety and commercial viability must be preserved. Thus an aircraft with significant subsurface damages along a wing or other structure should be grounded, and an aircraft having mere surface markings but no structural damages should be dispatched for flight. Trained NDI specialists and the equipment they may require are not typically immediately available at commercial aviation facilities. If specialists are to be summoned every time an impact site along the exterior of an aircraft is noticed, flight delays will occur, and passengers may need to be re-routed and possibly accommodated with hotel rooms and meals.
Thus, it would be advantageous to provide devices and methods for inspecting structures by modes requiring minimal training and interpretive expertise. Simplified methods in inspecting thin structures are needed. Methods are needed for detecting shallow damages in thicker structures, where shallow damages may indicate deeper damages. A need exists for rapid results providing go and no-go indications in NDI inspections.